As electronic devices has become smaller and lighter in weight with higher performance, materials for the electronic devices are required to have higher properties. One of methods for achieving such object is to make the materials as small as a nano-size level. In magnetic products formed by compacting magnetic powder, the use of fine magnetic powder is expected to improve soft or hard magnetic properties.
In magnetic recording tapes having hard magnetic particles coated on substrates, for instance, both the reduction of a magnetic particle size and the improvement of magnetic properties are required to increase their recording density. The magnetic particles have conventionally been ferrite powder, which suffers from low signal intensity because of small magnetization. To obtain sufficient output, magnetic metal particles of Fe and/or Co are suitable. However, when the particle size of metal particles is made smaller than 1 μm or less for higher recording density, the oxidation reaction of metal particles vigorously occurs in the air because they are vulnerable to oxidation, resulting in the deterioration of magnetization.
To improve the oxidation resistance of fine metal particles containing Fe and/or Co, proposals were made to coat the fine magnetic metal particles with ferrite layers (for instance, JP 2000-30920 A), or to coat Fe powder with graphite (for instance, JP 9-143502 A). However, the metal oxide coating disclosed in JP 2000-30920 A is disadvantageous in considerably oxidizing the metal particles, The coating of metal particles with graphite as disclosed in JP 9-143502 A needs a heat treatment at as high a temperature as 1600° C. to 2800° C. to melt carbon, disadvantageous for industrial use.
When magnetic metal powder is used in the form of moldings particularly in high-frequency applications, electrical insulation should be secured between magnetic metal particles to improve properties. For this purpose, each metal particle should be coated with a high-resistance material.
Proposed as a method for solving these problems is to coat metal particles with high-resistance boron nitride (BN) (see International Journal of Inorganic Materials 3 2001, p. 597, 2001). BN is a material usable for crucibles, having a melting point of 3000° C. excellent in high-temperature stability, low reactivity to metals and good insulation. The coating of metal particles with BN can be carried out by (1) heating a mixed powder of a metal and B by arc discharge in a nitrogen atmosphere, (2) heating a mixed powder of a metal and B in a mixed atmosphere of hydrogen and ammonia, or (3) heat-treating a mixture of a metal nitrate, urea and boric acid in a hydrogen atmosphere.
However, the above methods (1)-(3) suffer from the following disadvantages. Specifically, the above methods (1) and (2) suffer from the risk of burning due to rapid oxidation when handling ultrafine metal particles of 1 μm or less. The method (1) suffers from low productivity because of arc discharge, and is disadvantageous for industrial use because of high reaction temperatures near 2000° C. The production method (3) is likely to generate a harmful gas (NOx) because of the thermal decomposition of metal nitrates. In addition, the hydrogen gas used in the methods (2) and (3) is easily exploded, unsuitable for industrial use. These methods (2) and (3) suffer from extremely low productivity.
In addition, conventional coated metal particles have deteriorated saturation magnetization because part of metal particles are modified with coating materials. Thus, it is difficult to use fine particles produced by the conventional technologies for biochemical applications such as the extraction of DNA and proteins, magnetic recording media, etc.
Recently, fine magnetic particles have become used in medical diagnosis and biological examination. For instance, the use of superparamagnetic metal oxide particles as carriers for binding nucleic acids is proposed (JP 2001-78761 A). The superparamagnetic metal oxide exhibits magnetization only when an external magnetic field is applied. Because magnetic particles are exposed to acidic or alkaline solutions in the above applications, their surfaces should be chemically stable. In addition, antibodies for binding target substances should be easily attached to their surfaces. When magnetic powder is used as carriers for extracting nucleic acids, the metal or metal oxide powder is coated with silicon oxide (JP 2000-256388 A). According to this method, only coatings or fine particles of silicon oxide are used to cover the metal or metal oxide powder Silicon oxide is formed by the hydrolysis of silicon alkoxides or by using condensed sodium silicate.
The magnetic particles for magnetic beads should have as small a particle size as predominantly 1 to 10 nm to exhibit superparamagnetism. Accordingly, an extremely small force is induced in magnetic particles by an external magnetic field, failing to gather the particles efficiently. Because of a weak attraction force by a magnetic field, once gathered magnetic particles are likely to flow out together with a discharged solution. In the step of extracting nucleic acids with magnetic powder having a silicon oxide coating or fine silicon oxide particles on a magnetic metal core, the metal is likely to be dissolved in a solvent or oxidized, resulting in the deterioration of magnetic properties. The metal dissolved in a solvent forms a complex with a buffer solution, hindering the extraction of DNA. When the core is made of a metal oxide, the magnetic powder has extremely lower magnetic properties than when the core is made of a magnetic metal, resulting in lower efficiency in the extraction of nucleic acids.